Telephone system



v Q 1. k J Q33 IL k #6 1 @MW JR a M G.HECHT' TELEPHONE S'YSTEM FiledNov. 29, I940 5 b l H qmNEx-Bq 3* v .5 v Em r fi a U k v wmmmm WG 6 T Q,2: Q |1 V gg $.i bin B twkhsht U I a? [Ill-II Q" March 24, 1942;

man I 1 I van h E L: 2. .H n .1 1 7 .L n\ n m In I w v 1 u (a R 1:.mx 1. $53 3 53; m m 5.5 J35 ESE R moi I U U LU Ill U 3 U D in. Dmokumzzou QR 933 ATTORNEY other reeds of Patented Mar. 24, 1942TELEPHONE SYSTEM George Hecht, Astoria,

Laboratories,

York, N. Y., a corporation of New York Telephone N. Y., assignorto BellIncorporated, New

Application November 29, 1940, Serial No. 367,691 3 Claims. (Cl. 17917)This invention relates to telephone systems and more particularly to thereceiving equipment of a transmission line over which alternatingcurrents, preferably within Voice frequency 7 range, are transmitted foreffecting the operation of said equipment which, in turn, may be used tocontrol the selective positioning of switching apparatus. Moreparticularly, the invention is in the nature of an improvement in thereceiving equipment of the telephone transmission line disclosed inPatent-2,237,742 issued to A. A. Lundstrom on April 8, 1941. In saidpatent is disclosed a telephone system of the type in which automaticswitches are controlled by means responsive to alternating currents ofdifferent frequencies generated at a subscribers station and transmittedover a line terminating in a plurality of channel detectors eachselectively tuned to a different one of the signal frequencies andresponsive thereto to operate a relay which, in combination with a relayoperated over another channel detector responsive to a current ofanother signal frequency, serves to function a settable register, orsender, in which is locked-in a record of the digit indicated by the twooperated relays; that is, by the two frequencies transmitted from thesubscribers station.

In systems of this kind where the signal generating instrument isusually a plurality of keys or the like which are operated to produce adifferent combination of frequencies to which the channel detectors arerespectively tuned to respond, the signal energy available at the chan-.nels is a function of the ohmic resistance of the line, and since forthe best operating efficiency of the channels the quantity of energyapplied thereto must be kept within definitely controlled limits, itbecomes necessary to control the energy input to the channels by meansof a volume limiting device whose gain or loss is controlled by aninstrumentality responsive to the electrical characteristics of theline.

Accordingly, one feature of the present invention relates to a volumelimiter whose gain is controlled by a special frequency generated at thecalling station and transmitted to the volume limiter over the line loopestablished between the station and the central ofiice. In its specineapplication to the present embodiment of the invention, the transmittingdevice which generates the signal frequencies includes a special reedwhich is operated in combination with the device reserved for digit orcharacter signal purposes, and whenever one of the keys designating adigit or character is operated, the special reed is plucked to cause thegeneration of a current of the frequency indicated by its own naturalperiod, which current is then utilized at the central ofiice to set theain of the volume limiting device.

In accordance With another feature of this invention as applied to atelephone system in which a plurality of stations are located onthe'same line, the signal frequency'used for the purpose of setting thegain of the volume limiter is also utilized to identify the station fromwhich the signals are transmitted, there being one special frequency foreach station, each of which, however, is a1so-capable of setting thegain of the volume limiter. A receiving channel for each of the specialfrequencies is provided in parallel with the receiving channels for theother signal frequencies-so that, by the operationof the channelreserved for the individual stations, a registration of the callingstation designation may thus be effected in response thereto.

This and other features of the invention will be more readilyascertained from the following detailed description of said invention,appended claims and attached drawing in which is shown schematicallycertain elements of the telephone system more completely disclosed insaid abovementioned patent to A. A. Lundstrom and in which certain otherelements to which my invention more directly relates, are shown indetail.

Referring now, to the drawing, A, B, C and D represent party linesubscriber stations in an automatic telephone system, each of saidstations being equipped with a device I suitably arranged to produce oneor more currents of different frequencies preferably in the voicefrequency range, in response to the depression of keys or the like thatrepresent digits or other characters making up the Wanteddesignation.Each of the devices I has a special reed l which, when deflected andreleased, generates a frequency that characterizes the particularstation of which it forms a part and is plucked in combination with aplurality of other reeds in the device I whenever the same is operatedto generate signals indicated by the designation of the key used inoperating said device. 2, represents a line that extends from thestation instruments to the central ofiice whence, through one of aplurality of selectable sender connectors [3, the line is extended to anidle sender which comprises the coupling transformers 3, 4, 5 and 6,amplifiers 8 and 9, the volume limiter II, the enabler circuit I9 andthe channel detectors fl,

f f3, f f5 and IA, f3, f0, and D individually so designated inaccordance with the frequency to which each of the channels isrespectively assumed adapted to respond, which channels include, also,individually responding relays and the settable register relays (notshown) which follow the operation of the channel relays when operated tosignalize an incoming impulse.

The amplifiers 8 and 9 are commercially available circuit units, thedevice I is completely described in Patent 2,147,710 granted to R. F.Mallina on February 21, 1939, except as easily modified to include theadditional reed I while the channels JI-fD are completely disclosed inthe above-mentioned patent to A. A. Lundstrom already mentioned and towhich reference is made for a more complete understanding thereof. It isto be understood, however, that the invention is not limited to channelsof this description or to other apparatus herein used for the purposesof illustrating the operation of the invention.

In the operation of the invention, a call is initiated at any one of thestations, say, station E, by the removal of the station instrument fromthe switchhook whereupon the line is then extended by any suitable meansto the central ofice at which an available sender is then connected tosaid line through the medium of the sender connector I3 after which keytone is transmitted back to the calling station B by circuits not shown,as a signal that the sending device I may be operated to transmit thenumber of the wanted station and the designation of the calling stationall in the manner described in the abovementioned patent to A. A.Lundstrom.

When any one of the ten keys of the sending devices I is operated, twoof the five defined frequencies JI- S are produced in the combinationrequired to designate the digit or character represented by the keydepressed and also the special frequency B which is produced by thedeflection of reed I in combination with the two digit reeds. Theresulting current produced, which consists of two signal frequencies andfrequency B superimposed upon each other, is transmitted over a circuitwhich may be traced from one side of the sending device I over the lowerconductor of the looped pair 2, lower back contact of relay I2,

inner contacts of sender connector cut-in relay I 3, primary winding oftransformer 3, next to the inner contact of relay I3, inner back contactof relay I2, upper conductor of looped pair 2 to the other side of thestation sending device I. The current composed of the three frequenciesis induced in the secondary winding of transformer 3 and fed into theinput side of amplifier 8, whence, after suitable amplification, it isapplied from the output side of said amplifier to the input sides oftransformers 4 and 5, respectively, whence it is further applied,through transformer 4, to. the input side. of transformer I4 and,through transformer 5, to the input side of amplifier 9.

Considering that portion of the signal energy which passes throughtransformer 4, the energy further passes through the resistance network45 after which it is applied, through transformer I 4, to the grid ofvacuum tube 3|. Normally there is a plate current flowing through thistube as is obvious from the circuit drawing, the normal bias for thegrid thereof being drawn from negative battery 34 and resistances 35 and3B. When the signal impulse is transmitted, the bias on the grid isaltered in accordance with the value of the signal which, in turn,causes a variation in the plate current which is applied, throughtransformer 32, to the channel filters fI--J5 and also to the party linechannel filters fA--fB. Part of the energy, however, goes through theband-pass filter 3! and is applied, through transformer 38, to thecontrol electrodes of the cold cathode tube 39. The band-pass filter 31passes only the station identifying frequencies JA-,fD. However,although a station frequency passes through the band-pass filterregardless of its intensity, the tube 39 is not rendered conductingexcept if the level of the signal energy is above the predeterminedvolume limiting point. When this point is reached tube 39 is renderedconducting and thereupon applies a potential to condenser 40 which, inturn, applies a negative bias to the grid of vacuum tube 4| which, inturn, reduces the normal anode current flowing through the bridgecircuit comprising resistances 42, 43, 44 and the anode to cathoderesistance of said tube. This causes a potential difference to developbetween resistances 42 and 43 which is applied to the rectifier bridge33 causing direct current to flow therethrough and establish a shuntloss across the line to reduce the level of the incoming energy to thedesired point. The particular channels flf5 as well as channel fBoperate when the channel tubes are sensitized by the connection ofground to conductor I5 as described below, one or more channels jI-f5operating to register the called line designation and channel fBoperating to identify the calling station B. Inasmuch as the operationof the channels after sensitization is described in the above-mentionedpatent to A. A. Lundstrom, the operation of the channels beyond thispoint is not repeated except that reference is made to said patent for acomplete description of the operation of said channels.

Considering, now, that portion of the signal energy which is transmittedvia the transformer 6, it will be observed that enabler I0 comprises therectifier bridge I6 and the four polarized relays I'I, I8, I9 and 20,each having a permanently closed polarizing circuit through theirrespective lower windings which cause the armatures of the relays toassume the positions indicated in the drawing. Furthermore, between eachtwo relays, beginning with relay I1 and ending with relay 2!) there isan intermediate condenser-resistance network which joins a contact ofthe relay on the left with the upper winding of the relay to the right.Considering relays I1 and I8, for in stance, the network made up ofresistances 22 and 23 and condenser 2| joins the right contact of relayIT with the upper winding of relay I8. Since the armature of relay I1 isconnected to the positive battery 30, condenser 2| is normally chargedto the potential of this battery. Condenser 24 of the network betweenrelays I8 and I9, on the other hand, is normally at ground potentialsince it has ground applied to it through the left contact of relay I8.Contrariwise, condenser 29 of the network between relays I9 and 20 ischarged to the potential of the battery 30 through the upper winding ofrelay 20 and resistance 28 since the short-circuiting ground path forcondenser 29 is normally open at the right contact of relay I8 whichfact leaves condenser 29 exposed to the charging circuit that tracesfrom the positive pole of battery 39 through the upper winding of relay20, resistance 28 and condenser 29 to ground.

Assuming, therefore, that relays I'I, I8, I9 and 20 have theirrespective armatures in the positions shown and that condensers 2|, 24and 29 are charged, discharged and charged, respectively, as indicatedabove, ground is normally disconnected from conductor 15 at the rightcontact of relay 2!] which conductor is, in turn, connected to thecathodes of the channel detector tubes of the several channel detectors,fI--fD and causes them to be normally unresponsive to signal energyincoming through the transformer 32.

Now when a portion of the input energy is applied to the rectifier It,said energy is rectified and the current thereof is applied to the upperwinding of relay IT. The combined effect of the current now flowingthrough the upper winding of this relay and that always flowing throughits lower winding is such as to cause its armature to swing away fromits right contact. Since condenser 2| is normally charged to thepotential of battery 3!] over the armature and right contact of relay I!the opening of these contacts immediately causes the condenser todischarge to ground through resistance 23 and the upper winding of relayI8 to prolong the flow of current normally flowing through this windingof this relay. The direction of the discharge current as well as thenormal current through the upper winding of relay l8 opposes that of thecurrent normally flowing through its lower winding and since thedischarge current is initially of greater intensity through the propercontrol of the ohmic value of resistance 23, the release of relay [8 isdelayed for an interval after which the armature thereof is caused tobreak away from its left contact at a time subsequent to the operationof relay I! to engage its right contact. During the time that thearmature of relay I8 is engaged with its right contact, and rememberingthat condenser 24 is kept at ground potential while the armature of thisrelay is engaged with its left contact, then, as soon as the armaturebreaks with this contact, condenser 24 immediately begins to charge overa circuit which may be traced through resistance 26 and the upperwinding of relay l9 to battery 30, to continue to maintain the normalcurrent over this path for a short time subsequent to the operation ofrelay l8, whereupon the effect of the current through the lower windingof relay [9 continues to be neutralized for a short time by thischarging current. The armature of relay !9 is subsequently caused tobreak with its left contact at a time subsequent to the operation ofrelay [8, that is, at the time when the charging current for condenser24 through the upper winding of relay I9 is no longer powerful enough toovercome that flowing through the lower winding of the relay.

At the instant that the armature of relay I8 engages its right contactand before the armature of relay l9 leaves its left contact, a dischargepath is completed for condenser 29 which may be traced from groundthrough condenser 29, resistance 21, armature and contact of relay 59,right contact and armature of relay l8 to ground. The charge accumulatedon condenser 29 as a result of the charging current in the circuitthrough the upper winding of relay 20 is quickly drawn off through thelow resistance 21, and a current flow is quickly established through theupper winding of relay 20 for its rapid operation whereby ground isconnected to conductor to sensitize the detectors ,fI D so that when thearmature of the relay l9 breaks with its left contact, condenser 29 isin an uncharged condition and immediately starts recharging throughthe-upper winding of relay 20 to prolong the cur rent flow therethroughfor a subsequent period. This charging current overcomes the effect ofthat normally flowing through the lower winding of relay 20 and causesthe armature thereof to engage its right contact, thus applying groundto conductor l5 which, as said before, is connected to the cathode ofeach of the detector tubes in the channel detectors ;fI-fD. The channeldetectors are now made responsive and the three which are adapted torespond to the three frequencies in the input current that flows intheir respective input side via transformer 32 operate, causing in turn,the operation of their respective anode relays as fully described in theabove-mentioned patent to A. A. Lundstrom and which, in their turn,cause the operation of relays in the register to identify the digit orcharacter marked by two of the frequencies and the station designated bythe frequency B.

It is to be noted that relay 13 will make its right contact at a timesubsequent to theenergization of relay IT by the rectified currentflowing through the bridge I6. Ihis time is, of course, determined bythe discharge time of condenser 2| through resistance 23 which causesthe normal current through relay It to endure for a time after theoperation of relay H. At this time, relay l8 releases and closes itsright contact whereby condenser 29 is quickly discharged through the lowresistance 2'! and establishes a current through the upper winding ofrelay 20 to cause its immediate operation. This, as explained, placesground on conductor Hi to sensitize the conductors at some time afterthe signal is first applied. This delay is for the purpose of allowingthe unwanted spectrum energy in adjoining channels to dissipate itselfto avoid unwanted channel operation from this source. It is the purposeof condenser 24 to maintain, by its charge current through the upperwinding of relay IS, the normal current flowing therethrough to holdrelay l9 operated on its left contact for a time sufiicient to permitthe complete discharge of condenser 29 through resistance 21, leftcontact of relay l9, right contact of relay iii to ground. Relay l9 thenoperates to break its left contact and thereafter relay 2!] ismaintained on the charging current from battery 30, upper winding ofrelay 2%, resistance 28, condenser 29 to ground. When this dischargecurrent has dissipated itself, relay 2!] will operate and break itscontacts to remove ground from conductor I5 regardless of how longthereafter the energy is applied to relay IT by the signal.

The length of time during which the armature I of relay 20 engages itsfront contact to cause the channel detectors to remain activateddepends, of course, upon the constants of condensers 24 and 29 andresistances 25 and 28 which may be chosen to provide any suitableinterval desired. However, when condenser 29 is completely charged thecurrent through the lower winding of relay 29 again becomes effective,the armature of the relay breaks away from its right contact and groundis removed from conductor I5 and therefore from the cathode element ofthe several detector channels which, in consequence, will no longerrespond to any signal energy forthcoming over transformer 32 and therebycause their associated anode relays to release.

The circuit conditions prevailing in the enabler 10 during the presenceof signal energy through bridge H3 and after the armature of relay 20has disengaged its right contact are that relay I1 will have itsarmature disengaged from its right contact, relay I8 will have itsarmature making its left contact, while relay l9 will have its armaturemaking its left contact. This will in no way affect the charge oncondenser 29 or the condition of the armature of relay 20 which is outof engagement with its right contact. Hence it makes no difference howlong signal energy prevails upon the line subsequent to the time whenthe armature of relay 20 breaks away from its right contact, said energywill be ineffective to operate the channel detectors inasmuch as groundis disconnected from the cathodes of their several detector elements.

When the signal impulse is terminated, relay I! will restore to itsnormal condition, reestablishing quickly the charging circuit ofcondenser 2! and the circuits of the top winding of relay I8 to cause itto make its left contact. Relay l8 operates and causes the quickestablishment of the circuit through the top winding of relay l9 tocause it to quickly make its left contact. In this operation, condenser29 has not been charged since ground via the contacts of relays l8 and19 has not been removed. The enabler circuit is now ready for anothercycle of identical operations upon the reception of the next impulse.

While I have described my invention and the means for utilizing the samein connection with its specific application to a particular kind of atransmission line, it is to be understood that various otherapplications and embodiments thereof may be made by those skilled in theart without departing from the spirit of the invention as defined withinthe scope of the appended claims.

What is claimed is:

1. In a telephone system, a line, a resistance network connected acrosssaid line, a plurality of stations for said line adapted to generatealternating impulse signals characteristic of a wanted number and theidentity of the station, a plurality of channel detectors connected tosaid line, each of said channel detectors being adapted for individualresponse to each of the frequencies, respectively, of which said impulsesignals are composed, and means responsive to the frequency in thesignal which designates the station for causing signal energy to bedrawn through said resistance network whereby the quantity of signalenergy admitted into said channel detectors is reduced by the amountdrawn through said resistance network.

2. In a telephone system, a line, a plurality of stations thereon eachadapted to produce an alternating current signal characteristic of thestation, a resistance network connected across said line, meansconnected to said line responsive to a signal from one of said stationsfor registering the identity of the station, and means responsive to theintensity of said signal for causing a portion of the signal energy tobe drawn through said resistance network whereby the amount of signalenergy admitted into said first-mentioned means is reduced by the amountdrawn through said resistance network.

3. In a telephone system, a line, a plurality of stations thereon eachadapted to produce an alternating signal characteristic of the station,a plurality of channel detectors connected to said line each responsiverespectively to a frequency produced at a different one of saidstations, and a volume limiting device for controlling the quantity ofsignal energy admitted into said channel detectors comprising aband-pass filter for the passage therethrough of said differentfrequency signals, a gas-filled tube adapted to be rendered conductingwhen the signal level is at a predetermined level, a vacuum tubeincluding a first resistance bridge of which the anode circuit of saidtube constitutes one of its arms and having current normally flowingtherethrough, a second resistance bridge connected to said firstresistance bridge and across said line, and means responsive to theconductivity of said gas-filled tube for altering the current throughsaid first bridge to produce a difference of potential across theterminals of said second bridge whereby current flowing through thebridge in response to said difference of potential operates as a shuntto reduce the signal energy flowing into said channel detectors.

GEORGE I-lECHT.

